This invention relates to gasifiers and in particular to improvements in or relating to gasifiers whereby improved combustion may take place therewithin. The invention further relates to a method of operating a gasifier.
The production of producer gas (a mixture of about 30% carbon monoxide and 70% nitrogen, though other gases may also be present) is usually performed with a so-called gasifier, in which pyrolysis is performed. The gasifier comprises a chamber provided with air inlets in such a way that the burning of fuel within the chamber take place under controlled conditions. Originally, gasifiers used coal or coke as a fuel source, but interest in gasifiers has recently increased since they may be used for the disposal of various kinds of waste organic solid matter whilst yielding producer gas, which in turn may drive a combined heat and power unit, to yield both heat and electricity. Thus, the solid fuel may typically comprise wood or wood derivatives, straw, poultry litter, dried sewage sludge and refuse-derived combustible material, to mention but a few.
In a gasifier chamber, the solid fuel is reduced to a bed of carbon at a temperature of above 1000° C., a stream of air being passed through the bed with the combustion conditions set so that the oxygen in the air combines with the carbon to form carbon monoxide. Other gases such as methane and hydrogen may also be produced, depending upon the chemical composition of the fuel employed.
A product of the combustion in a gasifier is ash, but this is of relatively low density and of small volume compared to the solid fuel supplied to the gasifier. It is consequently relatively easy to dispose of, especially since it is wholly sterile. By contrast, the producer gas may be used for a variety of purposes, though since it is toxic in view of the carbon monoxide content, it must nevertheless be treated with care. For example, the collected producer gas may immediately be used in an internal combustion engine for the generation of electricity, without being stored for long periods or otherwise handled.
Though the principle upon which a gasifier operates is well known and understood, it is important that the combustion conditions are closely controlled in order that the production of carbon monoxide is optimised and that the carbon dioxide content of the producer gas is minimised. It has now been established that by controlling the actual introduction of air into the oxidation zone of a gasifier, it is possible to improve the gasifier performance, so increasing the overall efficiency of plant using this equipment.
Gasifier designs have been proposed in NL-A8900939 and CH-A-237348. In both of these prior designs, arrangements are made to introduce combustion air into a gasifier in such a way that the air is not directed strictly radially with respect to the axis of the combustion chamber, in an attempt to enhance combustion within the chamber. The present invention stems from attempts further to improve the production of producer gas.